There are known various types of such a glow plug having a combustion pressure sensor (hereinafter, may be referred to merely as a glow plug) (refer to, for example, Patent Document 1). FIG. 9 is a longitudinal, partially sectional view showing a simplified sectional structure of a glow plug 901 of the same type. The glow plug 901 of FIG. 9 is configured such that a rodlike (circular columnar) heater (e.g., a metal heater or a ceramic heater) 10 which generates heat through energization is disposed within a tubular housing 110 with its forward end (an end on a side toward a combustion chamber, or a lower end in FIG. 9) 10a projecting from a forward end 136 of the housing 110. Notably, the dashed lines in FIGS. 9 and 10 show the vicinity of a combustion chamber of an engine head. In the glow plug 901, in consideration of reception of combustion pressure, thermal expansion of the heater 10, etc., the heater 10 is disposed within the housing 110 in such a manner as to be displaceable in the direction of its axis G (axial direction), by a very small amount. That is, the heater 10 is disposed with a gap (annular gap) formed in cooperation with the inner circumferential surface of the housing 110. A sensing means; for example, a piezoelectric element 40, for detecting pressure generated as a result of combustion pressure pressing the heater 10 rearward from the forward end 10a is disposed rearward of the heater 10. This configuration is designed as follows: a force generated as a result of combustion pressure pressing the heater 10 rearward compresses the piezoelectric element 40, and an electric signal generated according to a change in compressive force applied to the piezoelectric element 40 is measured, whereby the combustion pressure is detected. Notably, in Patent Document 1, a strain gage is used as sensing means.
Incidentally, in the glow plug 901, there must be prevented entry of high-temperature, high-pressure combustion gas from the forward end 136 of the housing 110 into a rear interior region of the housing 110 through the above-mentioned annular gap between the inner circumferential surface of the housing 110 and the outer circumferential surface of the heater 10 (i.e., a seal must be established). In order to meet this requirement, usually a certain seal member is disposed in the annular gap between the inner circumferential surface of the housing 110 and the outer circumferential surface of the heater 10. For example, in the glow plug 901 of FIG. 9, as shown in the enlarged view in FIG. 9 and in FIG. 10, a diameter increased annular space K2 having a larger diameter is formed within a forward-end portion (forward-end housing 131) of the housing 110, and a seal member 60 for preventing entry of combustion gas into the rear interior region is disposed in the space K2. Meanwhile, in order to allow displacement in the direction of the axis G of the heater 10 in relation to the housing 110, a heat-resistant member having a sufficiently flexible annular film portion (membrane) 63 such as a metal membrane which is not bellows but is a diaphragm or the like which is readily deformable in the axial direction (e.g., a membrane of SUS630) is used for the seal member 60. The seal member 60 shown in FIGS. 9 and 10 has a small-diameter tubular portion 65 located on the forward side, a large-diameter tubular portion 61 located on the rear side, and the annular membrane portion 63 disposed between the small-diameter tubular portion 65 and the large-diameter tubular portion 61 in such a manner as to partition the diameter increased annular space K2 into axially forward and rearward parts. In the above-mentioned diameter increased annular space K2, the small-diameter tubular portion 65 of the seal member 60 is welded to the heater 10 at a predetermined position (solid triangular portion) W1 along the outer circumferential surface of the heater 10 by, for example, laser welding, and the large-diameter tubular portion 61 of the seal member 60 is fixed to the housing 110 at predetermined rearward positions (solid triangular portions) W2 and W3 along the circumferential direction by, for example, welding. This ensures a seal between the inner circumferential surface of the forward end 136 of the housing 110 and the outer circumferential surface of the heater 10.
Namely, in the case where the displacement of the heater 10 in the direction of the axis G (axial direction) in relation to the housing 110 is permitted by the seal member 60, the seal member 60 is configured such that mainly the annular membrane portion 63 deforms so as to allow the displacement of the heater 10 in the direction of the axis G while maintaining the seal. As described above, the seal member 60 plays a role of shutting off a high-temperature, high-pressure combustion gas which enters a space (annular gap) between the outer circumferential surface of the heater 10 and the inner circumferential surface of a portion of the housing 110 located near the forward end 136 thereof (hereinafter referred to as the “near-forward-end portion of the housing 110), and a role of allowing displacement of the heater 10 in the direction of the axis G in relation to the housing 110. Although unillustrated, according to a certain configuration, the seal member is disposed between the inner circumferential surface of the housing and the outer circumferential surface of the heater 10 at a position located deep in the housing away from the forward end of the housing, thereby establishing a seal therebetween. Such a configuration does not require provision of the aforementioned seal member 60 at the near-forward-end portion of the housing. However, depending on the position of the seal member disposed deep in the housing, there is usually required the provision, in addition to the seal member, of a holding member for holding the heater while allowing axial displacement of the heater, at the forward end of the housing or at the near-forward-end portion of the housing. That is, a configurational feature having easy deformability similar to that of the above-mentioned seal member must be provided. This is for the following reason: since the heater is in a loose fit condition in the housing, and the projecting forward end of the heater is a free end, in order to stably hold such a heater, the heater must be held (supported) at the forward end of the housing or at the near-forward-end portion of the housing, in addition to provision of the seal member disposed deep in the housing. As will be understood from this requirement, the above-mentioned seal member 60 also plays a role of holding the heater; therefore, the seal member 60 can also be said to be a holding member for holding the heater at the forward end of the housing or at the near-forward-end portion of the housing.